Audio transmission characteristic control circuit



P. o. FARNHAM 2,112,595

AUDIO TRANSMISSION CHARACTERISTIC CONTROL CIRCUIT Filed May 22, 1935 March 29, .1933.

O 0 0 O l .llAAAll "nun Patented Mar. 29, 1938 UNITED STATES PATET OFFICE Paul 0. Farnham, Boonton, N. J., assigner to Radio Corporation of America, a corporation of Delaware Application May 22, 1935, Serial No. 22,714

7 Claims.

My present invention relates to audio transmission characteristic control circuits, and more partioularly to a method of, and means for, limiting the band width of the audio output of a high fidelity radio re'ceiver.

It has been proposed in the past to apply automatic selectivity control to radio broadcast receivers, and particularly to receivers of the socalled high fidelity type. The latter brieiiy designates a receiver which is designed to reproduce in a substantially faithful manner the audio frequencies of the modulation envelope of the carrier. The purpose of using automatic selectivity control is to limit the amount of noise and unl5 wanted signals delivered to the reproducer.

Heretofore, this problem has been attacked by controlling the band-width of the normally wide band radio frequency characteristics of the signal selector networks; the narrowing being controlled only by the strength of the incoming carrier, and in inverse relation thereto, since reception of weak or distant signals is usually accompanied by a high noise level. The present invention, however, contemplates a control of the width of the 2 5 frequency band delivered to the reproducer, which control depends entirely, or for the most part, upon the actual amount of the noise level accompanying the received carrier. This control depending on noise level might conceivab-ly operate 30 on the selectivity characteristics of the tuned high frequency circuits, or alternatively upon the transmission characteristics of the audio amplier system.

Part of the automatic selectivity control func- 35 tion may be accomplished by an automatic control of band width with the strength of the incoming carrier level. When, however, a carrier input level is strong enough to override the noise level which is due directly to the radio receiver circuits 40 (thermal agitation and the like) it may, at times, be insufficient to justify the use of maximum band width when additional noise is present. Such additional noise may be static; man-made interference etc. Hence, it becomes necessary to regulate 45 the audio band Width delivered to the reproducer in response to the noise level at the audio network.

It may, therefore, be stated that it is one of the main objects of my present invention to use in a receiver an automatic control of band width trans- 50 mitted to the listener, the automatic control operating on the noise level which happens to. come through the receiver at any time whereby for little, or no, noise the full band width capabilities of the system are used while in the presence of 55 objectionable vnoise levels the transmission of the (Cl. Z50-20) higher audio frequencies is progressively reduced.

Another important object of the invention may be stated to reside in the provision of an audio transmission system wherein the efciency of the higher audio transmission is dependent upon the 5 noise level at the system input.

ther objects are to improve generally the efliciency of radio broadcast receivers, and more especially to provide receivers having noise control networks of improved and economical design 10 which are reliable in operation.

The novel features which I believe to be characteristic of my invention are set forth in particularity in the appended claims, the invention itself, however, as to both its organization and l5 method of operation will best be understood by reference to the following description taken in connection with the drawing in which I have indicated diagrammatically a circuit organization whereby my invention may be carried into effect.

In the drawing:-

Fig. 1 shows a circuit diagram of one form of the invention,

Fig. Z graphically illustrates the characteristics of the networks involved in the invention.

Referring now to the accompanying drawing there is shown in Fig. l a conventional radio receiver which embodies the invention, and it will be seen that the receiving system shown comprises a signal collector A; a radio frequency amplifier B; a detector, or demodulation, network C; an audio transmission network, comprising an amplifier D, followed by an audio output amplifier E; and the last amplifier E feeding any desired type of reproducer. The system may be of the superheterodyne type, or it may be a tuned radio frequency receiver. In the former case, the network B would comprise the usual tunable signal amplier, first detector, local oscillator, and intermediate frequency amplifier circuits. The symbol l would, then, denote the tuning condenser in the tuned preselector network 2 ahead of the intermediate frequency amplifier B. Of course, the network C would in such case be the second detector circuit.

If the receiver is of the tuned radio frequency type then the condenser l is variable, and acts as the variable tuning device of the set. The amplifier B may comprise one, or more, stages of tuned R. F. amplification. For the purposes of this application, and by way of illustration, let it be assumed that the receiving system shown is of the superheterodyne type, and that the input circuit 2 is resonated to the received carrier signal,

In order to maintain the signal I. F. amplitude at the input of demodulator C substantially uniform over a wide range of received carrier amplitude variation at collector A, there is employed an automatic volume control network (AVC in the drawing). Such `an AVC arrangement is well known; the signal input to the demodulator is usually branched off and rectified. The rectified current is employed to bias the controlled ampliers in the same sense as the variation of carrier amplitude. Such an arrangement is shown, for example, by Stuart Ballantine in application Serial No. 376,163, iiled July 5, 1929, Patent No. 2,046,237, June 30, 1936; and may be used for the present Purpose- The automatic selectivity control device, which has been previously referred to, is symbolically represented because its details are not essential to an understanding of the present invention; a circuit of the selectivity control type is disclosed byv G. L. Beers in application Serial No. 588,909, filed January 26, 1932, Patent No 2,017,523, October 15, 1935.

It will be suiiicient for the purposes of this application to point out that as the received carrier amplitude increases, the receiver selectivity is made suiiiciently broad to pass a modulation band of 10 k. o. or greater in order to accommodate the entire useful audio frequency modulation range. When weak, or distant, stations, are received the receiver selectivity is increased by the action of the selectivity control.

This is done because for Weak carrier levels the noise level is great enough to become distinguishable when the demodulated signals are reproduced. By rendering the tuned circuits more selective with weak received carriers the radio frequencies corresponding to the higher audio frequencies are eliminated, and thus the noises are prevented from being reproduced. It will be understood, cf course, that the particular type of selectivity control system shown is not essential to the functioning of the present invention.

The present invention contemplates an entirely.

different type of selectivity control.; one which is responsive to the audio output of the demodulator, and which is independent of the carrier amplitude variations. For this reason the details of the automatic selectivity control device are not given. In fact it is pointed out that the selectivity control may be manual in nature if desired. For example, a device disclosed by me in U. S. P: 1,945,427 of January 30, 1934; or one disclosed by myself and H. F. Argento in U. S. P. 1,943,789 of January 16, 1934, lmay be employed for varying the selectivity characteristic of the receiver.

It may happen, however, that While a received carrier input level is sufficiently strong to override the noise level which is due directly to the receiver circuits (thermal agitation and the like) such carrier levels may at times be insucient to warrant the use of a maximum band width because of additionalnoise which is due to static; man-made interference, etc. For this reaso-n the present invention contemplates the employment of an automatic control for limiting the band width capabilities of the audio frequency network of the receiver when the noise level is still high in spite of the operation of the A. S. C. system. This is accomplished by rectifying the higher audio .frequency currents above a predetermined maximum reproducible audio frequency limit, and utilizing the rectified currents to decrease, the transmission efficiency. oflv the higher audio frequency portion of the reproducible audio frequency range.

The noise control network comprises three essential elements. The filter circuit 5 is of the band pass type, and is conventionalized in disclosure. Those skilled in the art are w-ell aware of the fact that such lter circuits are readily designed in accordance with design principles sho-Wn in U. S. P. 1,227,113 of May 22, 1917 by G. A. Campbell.v Fig. 2 is that of circuit 5; it will be noted that it transmits audio frequency currents between 8000 cycles and 9000 cycles and suppresses all those outside these limits. The audio amplifier network D-E is designed, by well-known principles, to have the characteristic denoted by curve L in Fig. 2. Thus, it will be seen that the audio network is designed `to have a maximum reproducible audio frequency limit of 8000 cycles.

The output of lter circuit 5 is impressed on a rectifier 6 including in circuit therewith a resistor 'l properly by-passed. The direct current voltage developed across resistor 'I is used to regulate the audio network band width. As the intensity of the currents nowing through circuit 5 increases, the voltage across. resistor l increases. The increase in direct current voltage is caused to progressively reduce the transmission ofthe higher audio frequencies through network D-E.

This may be accomplished in many ways. n For Y example, a variable electronic impedance may be used in the manner disclosed by G. L. Beers in U. S. P. 1,961,329 of June 5, 1935. As disclosed in the latter patent, av tube l0 has its plate connected through audio frequency condenser H to the signal grid of tube E. 'Ihe cathode of tube I0 is grounded through bias network I3; the grid of tube l0- is connected by lead i4 to a point on resistor I which is made positive with respect to ground reference potential by action of the reclflel 6;

By having the network I3 designed to normally bias the grid of tube I0 to cut-off, the production of voltage across resistor I will result in decreasing the impedance of tube l0. Asa result the ilow of the higher audio frequency currents through path H-|0 to ground will increase; The dotted line curves in Fig. 2 show the progressive audio frequency transmission as the output of circuit 5 increases. The automatic noise control unit 5-6-|0 is shown constructed to operate on the noise energy in a, band of audio frequencies of from 8000 cycles to 9000 cycles which lies just outside the maximum audio-limit which it is desired to reproduce from the speaker.

Since the audiov circuit D-E passes substantially all frequencies up to 8000 cycles it is desiredto prevent nolsefrequencies within that range from reaching the reproducer. Now, since noise associated with the carrier occupies a frequency band extending both above, and below, 8000 cycles it is proposed to detect the presence of any noise by utilizing only those noise frequency components above 8000 cycles (circuit 5)', and

utilize the energy associated with these components for reducing the band width of the audio system so as to reduce the disturbing effect at the reproducer of those noise frequencies, also present, which are below 8000 cycles.

1t will be Seen that the essenuiu feature of this invention is the utilization of the noise energy in the higher portion ofthe audioV frequency range for regulating the reproducibleaudio band The characteristic curve N in I in a sense such as to preventY reproduction-r of 713 noises which may originate either within or outside of the receiving circuits.

While I have indicated and described a system for carrying my invention into effect, it will be apparent to one skilled in the art that my invention is by no means limited to the particular organization shown and described, but that many modifications may be made Without departing from the scope of my invention, as set forth in the appended claims.

What I claim is:

l. A method of regulating the transmission of noise impulses through an audio wave transmission system which includes ltering out from the audio waves said noise impulses, producing a direct voltage Varying in magnitude with the amplitude of said impulses, and utilizing said voltage for Varying the transmission efficiency of the higher audio frequencies through said system.

2. A method of regulating the transmision of noise impuls-es through an audio wave transmission system which includes filtering out from the audio waves said noise impulses, producing a direct volta-ge varying in magnitude with the amplitude of said impulses, and utilizing said voltage for Varying the transmission efficiency of the higher audio frequencies through said system in a sense to suppress said noise impulses.

3. In a radio receiver provided with a demodulator, reproducer and an audio transmission network, a bypass path for the higher audio frequencies connected to said network, and means, responsive solely to the amplitude of predetermined audio frequencies of the audio currents delivered by the demodulator, for controlling the current flow through said path.

4. In a radio receiver provided with a demodulator, reproducer and an audio transmission network, a bypass path for the higher audio frequencies connected to said network, and means, responsive solely to the amplitude of the higher audio frequencies of the audio currents delivered by the demodulator, for controlling the current ow through said path in a sense to prevent noise frequencies from reaching the reproducer.

5. In a radio receiver provided with a demodulator, reproducer and an audio transmission network, a bypass path for the higher audio frequencies connected to said network, means, responsive solely to the amplitude of the higher audio frequencies of the audio currents delivered by the demodulator, for controlling the current iiow through said path in a sense to prevent noise frequencies from reaching the reproducer, and means for varying the selectivity of the demodulator input circuit.

6. In a system as defined in claim 4, means for maintaining the signal amplitude at the demodulator input substantially uniform despite amplitude variations at the receiver signal collector.

7. In a radio receiving system; a detector, a signal selector network feeding the detector, an audio utilization network fed by the detector, means, responsive to carrier amplitude variations, for adjusting the selectivity of said selector network, and means, solely responsive to the amplitude of a predetermined band of undesired higher audio frequencies, for regulating the transmission through said audio network of the higher audio frequencies of the detected signals.

PAUL 0. FARNHAM. 

